LINE Platform sends POST {"events":[]} without an X-Line-Signature
header when the user clicks 'Verify' in the LINE Developers Console.
Both webhook.ts and monitor.ts rejected this with 400 'Missing
X-Line-Signature header', causing verification to fail.
Now detect the verification pattern (no signature + empty events array)
and return 200 OK immediately, while still requiring valid signatures
for all real webhook deliveries with non-empty events.
Fixes#16425
4f2f641950